A process for producing an electrically conductive organic polymer containing electrolyte ions as a dopant and having an electrical conductivity of 10.sup.-6 S/cm or more is already known which comprises subjecting aniline to chemical oxidative polymerization with the aid of a chemically oxidizing agent (U.S. Pat. No. 4,615,829). Further, there is described in JP-A-61-258831 that an oxidizing agent which can be used particularly advantageously in the above process for producing a conductive organic polymer through chemical oxidative polymerization is one whose standard electrode potential defined as the electromotive force in a reductive half-cell reaction using a standard hydrogen electrode as the standard electrode is 0.6 V or more. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, since electrically conductive organic polymers generally are insoluble and infusible, they cannot be formed into films by casting, and this constitutes a serious obstacle to the expansion of applications of conductive organic polymers. Although a film of a conductive organic polymer can be formed on an electrode by the electrochemical oxidative polymerization of aniline as described in JP-A-60-235831 and J. Polymer Sci., Polymer Chem. Ed., 26, 1531 (1988), this method is defective in that the electrode surface is the only place where the film can be formed and it is hence difficult to obtain a large-sized film, and that the production cost is high because the process utilizes electrochemical oxidation. A further drawback of the above film-forming method is that the film obtained is low in strength and is insoluble and infusible.
Various processes have, therefore, been proposed to date which comprise producing an intermediate soluble in organic solvents, casting a solution of the intermediate to form a film thereof, and then converting the intermediate into an electrically conductive polymer by a physical or chemical means. However, this method is defective in that it necessitates a high-temperature treatment or the conversion from intermediate to conductive polymer does not always proceed in accordance with the theory. Hence, such processes for producing a conductive organic polymer film are unpractical from both a production standpoint and the standpoint of the properties of films obtained.
In the field of polypyrroles or polythiophenes, polymers soluble in organic solvents are known. Such polymers can be obtained by a method in which a thiophene having a long-chain alkyl group as a substituent or a pyrrole having an alkanesulfonic acid group as a substituent is polymerized by means of electrochemical oxidative polymerization to give a poly(3-alkylthiophene) soluble in organic solvents or a poly(pyrrole-alkane-sulfonic acid) soluble in water, respectively. Either of these polymers can be formed into a film by casting a solution of the polymer. However, the above method is defective in that the production cost is exceedingly high because in either process, a special monomer is used and this monomer is polymerized by electrochemical oxidative polymerization.
On the other hand, in the field of the chemical oxidative polymerization of aniline, it has recently been reported that an organic solvent-soluble polyaniline can be obtained by the chemical oxidation polymerization of aniline using ammonium peroxodisulfate as an oxidizing agent in an amount of about 1/4 mol per mol of the aniline (A. G. MacDiarmid et al., Synthetic Metals, 21, 21 (1987); A. G. MacDiarmid et al., L. Alcacer (ed.), Conducting Polymers, 105-120 (D. Reidel Publishing Co., 1987)).
This polymer, however, has a low molecular weight as is apparent from the fact that it is soluble not only in N-methyl-2-pyrrolidone and dimethyl sulfoxide but in 80% acetic acid and 60% formic acid aqueous solutions. There is also described in the above literature references that a self-supporting film can be obtained from a solution of the polymer in N-methyl-2-pyrrolidone or dimethyl sulfoxide. Furthermore, there is also described that a conductive polymer film doped with acetic acid can be obtained from an acetic acid solution of the polymer and this film can be undoped with ammonia to eliminate the dopant. However, this undoped film does not fit for practical use, because its strength is low due to the low molecular weight of the polyaniline so that the film readily develops cracks upon bending.
It is also known that a polyaniline soluble in tetrahydrofuran can be obtained by oxidizing aniline with ammonium peroxodisulfate (J. Tang, Synthetic Metals, 24, 231 (1988)). However, this polymer also is thought to have a low molecular weight because it is soluble in tetrahydrofuran.
The present inventors made intensive studies to obtain a high molecular weight organic polymer soluble in organic solvents particularly by chemical oxidative polymerization of aniline. As a result, a quinonediimine-phenylenediamine type polyaniline was found which had a far higher molecular weight than the conventional polyanilines and which was soluble, in its undoped state, in various organic solvents despite its high molecular weight (JP-A-3-28229).
By use of a solution prepared by dissolving this polyaniline in an organic solvent, a free-standing polyaniline film can be obtained or a polyaniline film can be formed on a suitable substrate. It is also possible to obtain a conductive polyaniline film from such a film by immersing either of the above films in a protonic acid having a pK.sub.a value of 4.8 or less or in a solution of such a protonic acid thereby to dope the polymer with the protonic acid.
Further studies of the above-described quinonediimine-phenylenediamine type polyaniline (hereinafter often referred to as "oxidized-type polyaniline") which in its undoped state was soluble in organic solvents were made extensively by the present inventors. As a result, it was found that a polyaniline soluble in organic solvents even in its doped state could be obtained by reducing the oxidized-type polyaniline with a reducing agent to form an organic solvent-soluble imino-p-phenylene type polyaniline (hereinafter often referred to as "reduced-type polyaniline") and then doping this polyaniline with a specific electron acceptor (JP-A-3-52929).
Use of a solution containing such a doped polyaniline enables easy production of a conductive polyaniline film by casting which film not only has free-standing characteristics but is tough and flexible. Further, by casting or coating the solution on a suitable substrate, a conductive polyaniline film having toughness and flexibility can be formed on the substrate.
However, the above-described method for obtaining a conductive polyaniline has a disadvantage that it is useful only when an electron-acceptive dopant, i.e., an oxidizing dopant, is used as the dopant. In other words, the above method is not applicable to protonic acid dopants which do not have oxidizing ability.
The mechanism of the formation of the conductive polyaniline is as follows. When the reduced-type polyaniline contacts with an oxidizing dopant, an electron is drawn out from the unshared electron pair on a nitrogen atom in the polyaniline to yield a semiquinone radical and, as a result, the polyaniline comes to have electrical conductivity. Therefore, even if the reduced-type polyaniline is doped with a non-oxidizing protonic acid dopant, this only results in mere protonation of the polyaniline, so that a semiquinone radical is not yielded and the polyaniline remains electrically non-conductive